In satellite communication systems, SCR's on the surface of the earth receive messages transmitted from orbiting satellites. An SCR is able to receive its messages if the system knows where the SCR is located and can, therefore, cause the messages to be transmitted by satellites whose beams illuminate a Message Delivery Area (MDA) where the SCR is located.
When an SCR roams, it can move from one MDA to a relatively remote MDA, as when moving from the midwestern region of the United States to Japan. Unless the SCR's user notifies the communication system that the SCR has moved to a different MDA, the system will continue to send messages to the SCR using satellites whose beams illuminate the SCR's previous MDA. All such messages will be lost, and the user will be unaware that he has missed any messages. It is important, therefore, to ensure that the communication system is notified of any change in location that is large enough to cause an SCR to be moved to a different MDA.